1. Field of the Invention
The present invention relates to a dynamic DC offset removal apparatus provided for a receiver in a radio communication and a dynamic DC offset removal method, and more particularly, to a dynamic DC offset removal apparatus and dynamic DC offset removal method mainly used for a receiver in a digital mobile communication mounted with a direct conversion scheme radio section.
2. Description of Related Art
Due to influences of multipath fading which may occur in a radio channel, a receiver in a digital mobile communication receives radio waves through a plurality of paths of different propagation times simultaneously. For this reason, delay distortion occurs in the reception signal and a bit error rate characteristic deteriorates. Since the delay distortion increases as a symbol rate increases, it will no longer be possible to ignore the influences of delay distortion in high-speed digital mobile communication devices in the future and it is becoming indispensable to remove the delay distortion.
An equalizer is a typical means for correcting the delay distortion. In a radio communication, equalizers are conventionally mounted in receivers which carry out high-speed transmission, and in recent years equalizers are also mounted, for example, in cellular phones of the GSM (Global System for Mobile Communications) which is the digital cellular phone standard in Europe. There are also a growing number of receivers which estimate a DC offset component which may occur mainly at a radio RF section by a mean value of reception signals and remove the DC offset component as pre-processing of the equalizer and thereby improve equalization accuracy and the bit error rate characteristic.
FIG. 7 is a block diagram showing a configuration of a receiver having a conventional DC offset removal apparatus and FIG. 8 illustrates the configuration of a DC offset compensation processing section therein. In FIG. 7 and FIG. 8, receiver 10 having a DC offset removal apparatus is configured with DC offset compensation processing section 11, equalization processing section 12 and decoding processing section 13. DC offset compensation processing section 11 is configured with mean value calculation section 14 and DC offset removal section 15. DC offset compensation processing section 11 estimates a DC offset component using a mean value of reception signals through mean value calculation section 14 and removes it through DC offset removal section 15.
In recent years, for the purpose of reducing the circuit scale (cost reduction) of a radio RF section, a direct conversion reception scheme is being adopted instead of a heterodyne reception scheme which converts a reception frequency to an intermediate frequency. However, with the introduction of the direct conversion reception scheme, in the narrowband digital modulation scheme used in the GSM or the like, a DC offset component remains in a receiver detection output (baseband reception signal) due to the fact that a reception signal frequency is equal to a local transmission frequency, in particular, a drastic increase (hereinafter referred to as “dynamic DC offset”) of the DC offset occurs in the middle of a burst in the GSM, which is hardly removable by mean value estimation of the reception signals and may deteriorate the bit error rate characteristic.
To remove the dynamic DC offset, a conventional DC offset removal algorithm maps a digital baseband signal onto an IQ diagram, and determines I and Q coordinates of the center point by a two-dimensional fitting of geometric graphics using subsets of signal values. It is possible to capture a change of the dynamic DC offset by a change of the center point (for example, see Japanese Patent Application Laid-Open No. 2000-278335).
However, such a conventional DC offset removal apparatus has a problem that performing such arithmetic operations to determine a center point for arbitrary continuous subsets of the digital baseband signal causes an increase in the amount of processing.